Abstract
The Vanuatu region of the southwest Pacific comprises a complex 3D plate boundary system that has developed within the last 20 Ma owing to the opening of the North Fiji Basin and asymmetric rollback along the New Hebrides Trench. The southern Vanuatu region is characterised by a modern triple junction between the New Hebrides Trench, the backarc-spreading centre of the North Fiji Basin, and the Hunter Fracture Zone. At this intersection, the Hunter Ridge (a former magmatic arc) is being pulled apart by the southernmost propagating tip of the backarc Central Spreading Ridge of the North Fiji Basin. Extension of the Hunter Ridge lithosphere is being accommodated by the formation of a magmatically active rift zone referred to as the Hunter Ridge Rift Zone. Three distinct subduction-related melt types were identified within the olivine-hosted melt inclusions from a primitive olivine- and clinopyroxene-phyric volcanic rock from the Hunter Ridge Rift Zone, one of which has adakitic affinities. The primary nature of these melts indicates that we may have sampled the initial stages of mantle partial melting within a young and anomalously hot subduction environment. The preservation of geochemical diversity of the trapped melts suggests complex melting regimes influenced by different proportions of three enriched subduction- and mantle-derived silicate melts of possible slab, sediment and backarc origins, under tectonic conditions that facilitate the rapid ascent of newly formed melts. The three-dimensional geodynamics of this triple junction may play an important role in the generation and preservation of the unique and rare types of primitive magmas at the Hunter Ridge Rift Zone, by facilitating the interaction between backarc and arc magmatic processes, due to complex mantle flow including toroidal flow around the New Hebrides slab edge, upwelling at the slab edge associated with the return flow, and upwelling from backarc spreading at the Central Spreading Ridge.
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